The present application relates generally to methods and apparatuses for measuring changes in length or position; more particularly it relates to displacement measuring interferometry, where the phase of a Doppler signal is measured, and phase changes are accumulated to provide a measurement of the displacement (or relative position) of the motion being measured.
The use of interferometry to measure changes in position, length, distance or optical length is well known, see for example “Recent advances in displacement measuring interferometry” N. Bobroff, Measurement Science & Technology, pp. 907–926, Vol. 4, No. 9, September 1993, “High-resolution, high-speed, low data age uncertainty, heterodyne displacement measuring interferometer electronics” F. Demarest, Measurement Science and Technology, pp. 1024–1030, Vol. 9, No. 7, July 1998, U.S. Pat. No. 4,688,940 issued Aug. 25, 1987, and U.S. Pat. No. 5,767,972 issued Jun. 16, 1998.
A typical displacement measuring interferometer system consists of a frequency-stabilized light source, interferometer optics and measuring electronics. The interferometer optics split the laser light into a reference path and a measurement path, then recombines the light returning from the two paths and directs the recombined light to a photodiode where it produces an interference signal. A distance change of one wavelength in the measurement path relative to the reference path produces a phase change of 2π radians (360 degrees) in the interference signal. The measuring electronics measure and accumulate the change in-phase of the interference signal and provide a position output for the application.
Many interferometer applications, such as step-and-scan photolithography tools used to manufacture integrated circuits, require measuring multiple axes of motion at high velocity and with high resolution. An advanced photolithography system may include measurement of eight or more axes. The number of axes and limitations on the amount of light available from a common light source combine to limit the amount of light available for each measurement axis. Low light levels results in increased measurement noise or uncertainty.
Noise in the phase measurement may result in “glitches”, e.g., spurious measured position changes that are multiples of 2π when the measured signal levels are low. These glitches occur when an instantaneous noise value results in detection of an extra edge transition, or a phase change greater than 2π, that is accumulated as an actual change in position. These glitches may prevent useful measurement results, or in a production process may result in damaged or defective products.
Prior art methods to reduce glitches typically rely on filtering and/or other processing of the electrical measurement signal. For example, see U.S. Pat. No. 5,608,523.